A variety of techniques are used to recover nonferrous base metals from base metal ores. For some ores, the base metals of interest are directly leached from the ore into a leach solution, and the base metals are then removed from the leach solution to recover base metal values. For example, some copper ores are processed by dump leaching. Dump leaching involves leaching copper from the ore into an acidic leach solution. The copper is then recovered from the leach solution, such as by solvent extraction and electrowinning. If present, other base metals dissolved into the leach solution may be recovered before or after removal of the copper. For example, if the leach solution contains an appreciable quantity of dissolved zinc, the zinc may be recovered, following recovery of the copper, by solvent extraction or selective precipitation.
One problem with dump leaching is that the process results in a low recovery of copper when the ore contains significant amounts of copper that are not in an acid soluble form. Consequently, the dump leach process is normally limited to use on ores with appreciable quantities of acid soluble copper. Also, the solvent extraction process is expensive and not economic for use with many ores.
Another method of recovering base metals from base metal ores is to prepare a base metal ore concentrate by flotation and then further process the base metal ore concentrate, such as by smelting. During the flotation, it is important to produce a concentrate of sufficiently high grade for the smelting operation without losing excessive quantities of the valuable base metal to the flotation tail. For some ores, there is an added complication when significant quantities of the valuable base metal in the ore are in a form that is soluble in the flotation liquid. During flotation, such soluble base metals are susceptible to dissolving into the flotation liquid and then precipitating in a non-floatable form, such as a nonfloatable oxide or hydroxide precipitate. This nonfloatable precipitate is then lost to the flotation tail, representing significant loss of base metal value. One approach to address this problem is to pre-leach the ore to dissolve the soluble base metal, and separately remove the soluble base metal from the leach solution. For example, a copper ore may be pre-leached with water or with an acidic leach solution to dissolve soluble copper. The soluble copper is then recoverable from the leach solution by solvent extraction and electrowinning, in the same way as previously discussed for dump leach operations. However, the additional pre-leach process is expensive and is often not economical.
Another proposed method to recover base metals is pressure oxidization, especially when a significant portion of the base metal value is contained in sulfide minerals. During pressure oxidation, a slurry of the ore is subjected to high temperature and high pressure in an autoclave in the presence of oxygen gas, causing decomposition of sulfide minerals and permitting most or all of the base metals of interest to dissolve into aqueous liquid of the slurry. For example, this process has been proposed especially for processing copper ores that contain a substantial amount of primary copper sulfide minerals, such as chalcopyrite. One consideration for pressure oxidation as a processing option is that it is expensive. Also, it is often desirable to first prepare a sulfide concentrate from the ore prior to pressure oxidation, to increase sulfide sulfur content in the autoclave feed to provide sufficient fuel for the pressure oxidation. If the ore contains significant quantities of base metal in soluble form, there is a potential for significant losses of base metal values during flotation, in a manner as discussed above. The soluble base metal content could be removed prior to flotation by pre-leaching of the ore, but as noted above such pre-leaching is expensive and often not economical.